There are many examples of processes where liquid coatings are applied to the surface of a medium, and where it is necessary to remove a volatile portion of the liquid coating by some drying process. The image-wise application of aqueous inks in a high speed inkjet printer to generate printed product, and the subsequent removal of water from the image-wise ink deposit, is one example of such a process. Web coating of either aqueous or organic solvent based materials in the production of photographic films or thermal imaging donor material and the removal of water or solvent from the coated web is another example. The drying process often involves the application of heat and an airstream to evaporate the volatile portion of the liquid coating and remove the vapor from proximity to the medium. The application of heat and the removal of the volatile component vapor both accelerate the evaporation process.
In pneumatic acoustic generator air impingement drying systems, there are generally three components that are used to accelerate the drying process. Heated air is supplied through a slot in the dryer so that it impinges on the coated medium. This heated air supplies two of the components that accelerate drying: heat and an airstream. A third component that is used to accelerate the evaporation of volatile component of the liquid coating is the acoustic energy. The pneumatic acoustic generator is designed such that it generates acoustic waves (i.e., sound) at high sound pressure levels and at fixed frequencies as the impinging air stream passes through the main air channel of the pneumatic acoustic generator. The output of the pneumatic acoustic generator is an airstream that contains high levels of sound energy. The pressure fluctuations associated with the sound energy will disrupt the boundary layer that forms at the interface between the liquid coating and the air; this allows an accelerated transport of both heat and vapor at the liquid to gas boundary. In the absence of the pressure fluctuations associated with the sound energy, the transport of vapor across the boundary layer would rely on diffusion.
To be most efficient, the drying system needs to not only supply the air impingement stream for drying but also provide a means of removing that air from the air impingement drying region after it has collected volatile vapor from the coating. An air exhaust system is generally provided to remove air from the drying region. This exhaust air is typically heated to higher temperatures than components of the apparatus that are outside the drying system, and it carries significant quantities of water or solvent vapor generated during the drying process. If this hot, vapor-carrying air comes into contact with cooler components of the apparatus, the vapor may condense on those components. Condensation may collect to the point that it forms drops that may fall onto the medium that is being dried, thereby producing coating artifacts or image artifacts that are unacceptable. It would be advantageous to control the impingement and exhaust airstreams so that escape of the hot, vapor-laden-air from the drying system is not possible.